Janus kinase (JAK) is a non-receptor type of tyrosine kinases (PTKs), which resides in cells and transduces cytokine stimulation signal via JAK-STAT pathway. By JAK-STAT pathway, a chemical signal outside the cell is transduced into a gene promoter on endonuclear DNA through cell membrane, and finally affects the DNA in cell to change its transcription and activity level. JAK-STAT pathway mainly consists of three components: (1) a receptor; (2) Janus kinase (JAK) and (3) a signal transducer and activator of transcription (STAT) protein. The receptor can be activated by interferon, interleukin, growth factor or other chemical messenger, and such activation leads to the phosphorylation of JAK itself. Then, the STAT protein bonds to the phosphorylated receptor, so that STAT is phosphorylated by JAK. After that, the phosphorylated STAT protein is isolated from the receptor, then dimerized and translocated into cell nucleus, thereby bonding to specific DNA site and changing transcription (Scott, M. J., C. J. Godshall et al. (2002). “Jaks, STATs, Cytokines, and Sepsis” Clin Diagn Lab Immunol 9(6): 1153-9).
JAK family plays a role in the cytokine-dependent regulation of proliferation and function of cells involved in immune response. At present, there are four known mammalian JAK family members: JAK1, JAK2, JAK3 and TYK2 (Tyrosine kinase 2). The JAK proteins have a size ranging from 120 kDa to 140 kDa, and comprise 7 conserved JAK homology (JH) domains. One of them is a functional catalytic kinase domain, and another is a pseudokinase domain which effectively exerts a regulatory function and/or acts as a docking site for STATs (Scott, Godshall et al. 2002, supra).
At present, various Janus kinase inhibitors have been reported. The Chinese patent application No. 201410784461.2 with the filing date of Dec. 16, 2014 discloses several JAK inhibitors (the contents of which are incorporated herein by reference in their entirety), including (3R)-3-[3-amino-4-(7H-pyrrolo[2,3-d]pyrimidin-4-yl)-1H-pyrazol-1-yl]-3-cyclopentyl-propionitrile compound represented by formula I:

In addition to therapeutic efficacy, drug developers attempt to provide a suitable form of an active molecule having properties as a drug. From the viewpoint of obtaining a commercially viable production method or from the viewpoint of producing a pharmaceutical composition comprising an active compound, the chemical stability, solid-state stability and shelf life of an active ingredient are very important factors. Therefore, it is very important for the development of a drug to provide a suitable form of the drug having desired properties.